There presently exists numerous means to provide for data transmission some of which utilizes optical fibers. In present fiber optic data transmission links or systems they often transmit data which includes a long period of high or low state. Such data is normal when it is not coded in any way and the system may rest in either state.
When coded data is transmitted through the fiber, this usually involves a transmitter and an ac coupled receiver with additional circuitry to convert the data into pulses or other simple code to enable the receiver to detect the signal which is then converted back to the original form. However, this additional coding circuitry is usually complex and tends to restrict the use of the data link by reducing the modulation bandwidth and sensitivity of the receiver for effective operation.
The complexity of the coding circuitry could be reduced if the link could be made to respond to dc signals.
Several attempts have been made to fabricate a transmitter and receiver with a dc response. Some techniques which are employed use coding and decoding systems incorporated into the transmitter and receiver respectively. This leads to a complex and relatively expensive arrangement to effectuate such transmission, which is undesirable.
As shown in FIG. 1, direct current coupled transmitters and receivers may be used without the coding and decoding systems, however with present designs certain difficulties exist, especially when the range of input signal varies widely. The first difficulty is that of dc instability with time and temperature but this may be corrected through the use of operational amplifiers and careful design and may be overcome.
A second difficulty is more critical and involves data width distortion of the signal, as for example shown in FIG. 3. This distortion is essentially the difference in delay for low to high and high to low transitions resulting in narrower or wider pulses from the receiver than originally transmitted. Data width distortion with varying input signal levels limits the usefulness of a dc coupled link.
To avoid this, the receiver, which detects the different optical power levels, utilizes a comparator with its threshold ideally set at half the peak signal level so that data width distortion is not critical. However, the present techniques used to monitor the peak signal level and set the comparator threshold cannot operate for very long low states where the threshold control circuit is required to anticipate the following high state or peak signal level.